Removed redundant delayed-job tasks
[Delayed Job]: Re-try all jobs 3 times and set default_priority to 10 ...

Most delayed jobs involve mailing. And mailing should be low priority
and re-tryable

The few jobs that should be high priority - like compiling quizzes and worksheets
now have their priorities explicitly set
[Compile Quiz]: Passing just the quiz_id to delayed_job

Whatever is passed to delayed_job is serialized and stored in the DB.
Much better to pass something short and do any object retrieval in perform itself

Also, defined failure method for the task
[Compile Testpaper]: Passing id here too as is done for CompileQuiz